I am coming across as too quiet/introverted!

Hey, I just wanted to add that I think what they've said to you is far too personal. Where on any of the performance standards or lesson grading criteria does it say that you need to be extroverted, or have any other personality trait for that matter?

When I was a teacher I was told that my posture and my accent were both problems.

I feel for you, I think general vague comments like this are unhelpful and unkind.
However I think if it were me, I would say to the person who said this the following:' I am disappointed regarding the job because I had hoped to get it. I appreciate your feedback, but please could you give me a couple of specfic examples about how I could improve in these areas. Ask them when they are on their own. You may find that they give you some useful pointers, such as about getting involved in things, or perhaps some tips about aspects of teaching that could be improved, then you will have learnt something and you will be aware of what they perceive rightly or wrongly to be your areas of weaknesses. Although it might be awkward doing this might be better than just wondering.
Good Luck, chin up,

It seems to me to be an excuse. I would suggest it is far more likely you have made someone, somewhere uncomfortable, trodden on a toe, outshone someone, made someone look or feel less important/effective/dynamic than their self-image can bear but management had to make up some nonsense because they couldn't give you the real reason for not wanting your services in the future.

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I think you've uncovered what's really going on here. I really think management work a lot of the time on 'hunches', so it seems they have one about you, and have let it slip out. I'm worried that they will start to build up evidence that will back their hunch up now, in a sense that they know what they want to see now, so they will see it.